I splurged on a Generator recently and was trying to find a way to power the furnace.
Option one. Cut the line between the furnace and the panel, add an outlet on the panel side and a 3 prong cord end on the furnace side. Power goes out, unplug the furnace and plug directly into the Generator.
This is a fine idea and fully-allowable within the electrical code as long as the plug and socket selected are suitable. You may wish to use a twist-lock plug and socket so that A) the funance won't come accidentally unplugged, and B) so that it won't be possible for the plug to come partially out possibly allowing a conductive object to fall between the plug and the socket.
When you plug your generator into the furnace, use a heavy-duty (like #14 or, better yet, #12) extension cord and keep it as short as possible. You'll probably have to cut the cord to replace the standard end with a twist-lock, so cut it to the desired length plus maybe ten feet for slack. The wiring used in your house is probably #14, maybe #12. So, such an extension cord is as good as your house's own installed wiring.
Option two. Make a cord that goes from the 220 twist lock on my generator and has a 3 prong 220 on the other end. Unplug the dryer, cut the power to the panel and plug the Generator into the Dryer outlet, backfilling the panel and power to the house. This would take a 25 foot line and have the Generator out front of my house.
NO! NEVER DO THIS OR ANYTHING LIKE IT. Never, never plug your generator into an outlet hoping to "back feed" your house's electrical system. Your house's electrical system is not designed to work this way. Many a house fire have resulted from this old "trick."
Option three. Add a 220 circuit breaker in the panel, pull an outlet to the exterior wall above the panel on the back porch. Put an outdoor 220 outlet there. Make same cord as Dryer setup but now it's shorter,less than 10 feet, and located at the back door closer to the panel. This setup seems to be the best with the shortest cord and least amp loss.
This is only slightly less offensive than option #2 above. Slightly less offensive in that you'd install this new outlet with #4 wire. It'll still violate the electrical code to use an outlet and breaker this way.
Keep in mind that anything "jerry-rigged" will be a problem when you go to sell your house. You'll probably end up having to pay an electrician to undo it. So, the few bucks you save today will be eaten up entirely when you go to sell. The same thing is true on option #2 above. If it leaves a scorch mark inside your breaker panel and a home inspector working for a potential buyer see it, you'll end up having to pay a pro to replace the entire panel and that will cost a lot of money.
The best option is a purpose-built load switch properly installed. But, that can be expensive.
As I noted above, the wiring inside of your house is typically #14. If you use #14 or -- better yet -- #12, three-wire extension cords in good condition, this is as good as or better than the wiring in your house.
So, when the power fails, unplug the furnace from the wall outlet and run a #12 extension cord from where you set the generator up to where the furnace is. Route the cord so that it doesn't get stepped or or run over by cars or slammed in doors etc. Then, run several more #12 cords from the generator to your other loads, perhaps the refrigerator, the living room, etc. Each cord is good for about 15A, about 1800W on a 120V circuit. The generators I've seen that have 120V outlets have an individual 15A breaker on each outlet, so there's no danger of overloading here at all as long as you use #14 or #12 cords.
Insist on using a long, continuous cord from the generator to the immediate area of each load. Don't plug cords into cords into cords.
Keep your cords neatly on reels between use. This will protect them from damage and will also make it infinitely easier to set your system up in an emergency.
Replace any cord with any damage at all, any at all. Resist the temptation to just "put some tape on it." Home Depot gets $56 for a 100-foot, 12/3, indoor/outdoor-rated, heavy-duty extension cord; you can shop around and find it for less, I'm sure. Fifty bucks isn't worth burning your house down over.
You may also find that your furnace draws a considerable inrush and may not be able to start while the refrigerator or other major load is running. So, in an emergency, you may have to turn off other major loads like the frige, start the furnace and run the house of to maybe 75, then turn the furnace off and turn your other major loads back on and run that way until the temperature in the house falls to maybe 65 and then repeat. This is one of the misconceptions of a load switch, that you can just throw this one switch, start the generator, and everything works normally. No. The generator doesn't have the surge capacity that your utility company does. So, even with a load switch, you sometimes have to do this sort of load alternation.
Oh, and one more thing: Generators are noisey and they're also common theft targets. It's not hard for the thief to hear where a generator is. The common solution to this is to run the generator indoors, often in a garage. If the garage is detached, just know that you'll need to open the big door all the way and let the garage air out for a fraction of an hour or so before you go in. If the garage is attached to the house, well... this kills people every year; just don't do it. The better solution is to make sure you have a suitable length of logging chain (big, heavy chain) and a suitable lock and something sturdy to chain the generator to. If your back yard is fenced and you're planning to put the generator back there, having a provision to lock the gate -- something you may not normally do -- would be a good idea.
